This invention relates to semiconductor processing and, more particularly, to processing semiconductor materials using lithography. Specifically, one embodiment of the invention is directed to a multilevel resist process involving the use of a plated transfer layer useful for fabricating semiconductor devices, such as gallium arsenide (GaAs) FET gate fabrication.
Generally, there are two types of lithographic techniques used for semiconductor processing, optical and electron-beam (e-beam). E-beam lithography is a valuable tool for fabrication of devices with geometries below the limits of optical lithography. However, the need for a small beam diameter and multiple scans to produce a minimum geometry with good linewidth control greatly reduces throughput. Poor sensitivity of available resists also reduces throughput.
By minimizing sources of beam size variation and e-beam system noise, it is possible to expose and develop a layer of positive resist (PMMA) to yield a 0.25 micron opening directly. See, L. G. Studebaker, G. J. DeWitte, F. L. Bugely, D. H. Riehl, "Prototype to Production Using the Hewlett-Packard Quarter-Micron Electron Beam System," J. Vac. Sci. Technol. B 5(1),92,1987. However, this requires a very uniform beam size over the exposure field.
For example, a 0.05 micron variation in a 0.25 micron beam diameter represents about a 50% exposure dose variation if the beam current is constant. FIG. 1 shows how much this beam size variation affects opening width control in a single layer of PMMA resist.
In FIG. 1A, development contours are modeled using the SAMPLE program available from the University of California at Berkeley for a beam diameter of 0.25 micron and a dose of 80 .mu.C/sq. cm. The four contours are advancing with time to define an opening in the resist. Note that the contours are relatively far apart when the width of the opening is 0.25 micron, indicating that the process is difficult to control. Nonetheless, with suitable process control, a 0.25 micron opening is attainable.
However, with the addition of slight beam size variation, the process deteriorates. FIG. 1B shows the same four development contours for PMMA exposure with a 0.2 micron beam diameter. Since the beam area is smaller, and the beam current has not changed, the effective exposure dose has increased to about 120 .mu.C/sq. cm. This causes the resist to develop faster, resulting in a larger opening.
As shown by a comparison of FIGS. 1A and 1B, when the regions exposed with a 0.2 micron beam are opened to a width of about 0.25 micron, the regions exposed with a 0.25 micron beam are not yet open. Also, when the regions exposed with a 0.25 micron beam are opened to a width of about 0.25 micron, the regions exposed with a 0.2 micron beam are opened to about 0.4 micron. If the beam diameter is varying from 0.2 to 0.25 micron over the exposure field, FIGS. 1A and 1B represent the variation in opening width that should be expected on a wafer for a particular development time contour. This difference in linewidth versus nominal linewidth is plotted in FIG. 2 and graphically illustrates the result of a beam size variation when writing an opening in PMMA.
FIG. 3 shows the same modeled development time contours for a line of PMMA. A line is produced by exposing the entire field except where the resist is to remain. Note that development contours are relatively close together when the resist linewidth is 0.25 micron, indicating that the process should be easier to control. More importantly, the difference in the linewidth due to the beam size variation from 0.2 to 0.25 micron is smaller for a line of PMMA resist than for an opening, which means that the process is less sensitive to beam size variations. This difference in linewidth versus the nominal linewidth is plotted in FIG. 4 and shows that the linewidth variation is constant or decreasing for decreasing linewidth of PMMA resist.
The results of FIGS. 1 through 4 can be summarized as follows. Achieving a 0.25 micron opening in positive resist with a 0.25 micron beam size requires under-exposure and/or under-development, which leads to a large linewidth variation for a dose variation. Achieving a 0.25 micron line of negative resist using a 0.25 micron beam size permits over-exposure and/or over-development, leading to a small linewidth variation for a dose variation. The major drawback of producing a line of PMMA resist using e-beam exposure is that essentially the entire exposure field must be written, except for a few fine lines. This is a tremendous reduction of wafer throughput for the exposure tool compared to exposing a few fine lines.
A number of researchers are working on methods of producing smaller lines with e-beam lithography, as well as optical lithography. However, progress in this field is slow.